Re: [Xen-devel] Problem with commit bf22ff45bed664aefb5c4e43029057a199b7070c

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

On Mon, 10 Jul 2017, Juergen Gross wrote:
> On 07/07/17 19:11, Thomas Gleixner wrote:
> > On Fri, 7 Jul 2017, Thomas Gleixner wrote:
> > 
> >> On Fri, 7 Jul 2017, Juergen Gross wrote:
> >>
> >>> Commit bf22ff45bed664aefb5c4e43029057a199b7070c ("genirq: Avoid
> >>> unnecessary low level irq function calls") breaks Xen guest
> >>> save/restore handling.
> >>>
> >>> The main problem are the PV devices using Xen event channels as
> >>> interrupt sources which are represented as an "irq chip" in the kernel.
> >>> When saving the guest the event channels are masked internally. At
> >>> restore time event channels are re-established and unmasked via
> >>> irq_startup().
> > 
> > And how exactly gets irq_startup() invoked on those event channels?
> 
> [   30.791879] Call Trace:
> [   30.791883]  ? irq_get_irq_data+0xe/0x20
> [   30.791886]  enable_dynirq+0x23/0x30
> [   30.791888]  unmask_irq.part.33+0x26/0x40
> [   30.791890]  irq_enable+0x65/0x70
> [   30.791891]  irq_startup+0x3c/0x110
> [   30.791893]  __enable_irq+0x37/0x60
> [   30.791895]  resume_irqs+0xbe/0xe0
> [   30.791897]  irq_pm_syscore_resume+0x13/0x20
> [   30.791900]  syscore_resume+0x50/0x1b0
> [   30.791902]  xen_suspend+0x76/0x140
> 
> > 
> >>> I have a patch repairing the issue, but I'm not sure if this way to do
> >>> it would be accepted. I have exported mask_irq() and I'm doing the
> >>> masking now through this function. Would the attached patch be
> >>> acceptable? Or is there a better way to solve the problem?
> >>
> >> Without looking at the patch (too lazy to fiddle with attachments right
> >> now), this is definitely wrong. I'll have a look later tonight.
> > 
> > Not that I'm surprised, but that patch is exactly what I expected. Export a
> > random function, which helps to paper over the real problem and run away.
> > These functions are internal for a reason and we worked hard on making
> > people understand that fiddling with the internals of interrupts is a
> > NONO. If there are special requirements for a good reason, then we create
> > proper interfaces and infrastructure, if there is no good reason, then the
> > problematic code needs to be fixed. There is no exception for XEN.
> 
> I'm absolutely on your side here. That was the reason I didn't send
> the patch right away, but asked how to solve my issue in a way which
> isn't "quick and dirty". The patch was just the easiest way to explain
> what should be the result of the proper solution.

Fair enough!

> > Can you please explain how that save/restore stuff works and which
> > functions are involved?
> 
> It is based on suspend/resume framework. The main work to be done
> additionally is to disconnect from the pv-backends at save time and
> connect to the pv-backends again at restore time.
> 
> The main function triggering all that is xen_suspend() (as seen in
> above backtrace).

The untested patch below should give you hooks to do what you need to do.

Add the irq_suspend/resume callbacks and set the IRQCHIP_GENERIC_SUSPEND
flag on your xen irqchip, so it actually gets invoked.

I have to make that opt in right now because the callbacks are used in the
generic irqchip implementation already. We can revisit that when you can
confirm that this is actually solving the problem.

Thanks,

        tglx

8<----------------------
--- a/include/linux/irq.h
+++ b/include/linux/irq.h
@@ -476,6 +476,8 @@ struct irq_chip {
  * IRQCHIP_SKIP_SET_WAKE:      Skip chip.irq_set_wake(), for this irq chip
  * IRQCHIP_ONESHOT_SAFE:       One shot does not require mask/unmask
  * IRQCHIP_EOI_THREADED:       Chip requires eoi() on unmask in threaded mode
+ * IRQCHIP_GENERIC_SUSPEND:    Use the suspend/resume callbacks in
+ *                             device_irq_suspend/resume
  */
 enum {
        IRQCHIP_SET_TYPE_MASKED         = (1 <<  0),
@@ -485,6 +487,7 @@ enum {
        IRQCHIP_SKIP_SET_WAKE           = (1 <<  4),
        IRQCHIP_ONESHOT_SAFE            = (1 <<  5),
        IRQCHIP_EOI_THREADED            = (1 <<  6),
+       IRQCHIP_GENERIC_SUSPEND         = (1 <<  7),
 };
 
 #include <linux/irqdesc.h>
--- a/kernel/irq/pm.c
+++ b/kernel/irq/pm.c
@@ -70,6 +70,8 @@ void irq_pm_remove_action(struct irq_des
 
 static bool suspend_device_irq(struct irq_desc *desc)
 {
+       struct irq_chip *chip;
+
        if (!desc->action || irq_desc_is_chained(desc) ||
            desc->no_suspend_depth)
                return false;
@@ -94,8 +96,13 @@ static bool suspend_device_irq(struct ir
         * chip level. The chip implementation indicates that with
         * IRQCHIP_MASK_ON_SUSPEND.
         */
-       if (irq_desc_get_chip(desc)->flags & IRQCHIP_MASK_ON_SUSPEND)
+       chip = irq_desc_get_chip(desc);
+       if (chip->flags & IRQCHIP_MASK_ON_SUSPEND)
                mask_irq(desc);
+
+       if ((chip->flags & IRQCHIP_GENERIC_SUSPEND) && chip->irq_suspend)
+               chip->irq_suspend(&desc->irq_data);
+
        return true;
 }
 
@@ -138,6 +145,8 @@ EXPORT_SYMBOL_GPL(suspend_device_irqs);
 
 static void resume_irq(struct irq_desc *desc)
 {
+       struct irq_chip *chip;
+
        irqd_clear(&desc->irq_data, IRQD_WAKEUP_ARMED);
 
        if (desc->istate & IRQS_SUSPENDED)
@@ -150,6 +159,10 @@ static void resume_irq(struct irq_desc *
        /* Pretend that it got disabled ! */
        desc->depth++;
 resume:
+       chip = irq_desc_get_chip(desc);
+       if ((chip->flags & IRQCHIP_GENERIC_SUSPEND) && chip->irq_resume)
+               chip->irq_resume(&desc->irq_data);
+
        desc->istate &= ~IRQS_SUSPENDED;
        __enable_irq(desc);
 }



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